Carbon black has so far been used as a reinforcing filler for rubber. This is because carbon black has a high reinforcing property and an excellent abrasion resistance as compared with those of other fillers. In recent years, under the circumstances of socially demanding for saving energy and natural resources, particularly in order to save a fuel consumption of automobiles, a reduction in heat generation of a rubber composition has come to be required at the same time.
In the case of intending a reduction in a heat generation of a rubber composition by virtue of carbon black, it is considered to fill a small amount of carbon black or to use carbon black having a large particle size. In either method, however, it is well known that a reduction in the heat generation has an antinomic relation with the reinforcing property and the abrasion resistance.
On the other hand, silica is known as a filler for reducing heat generation for a rubber composition, and there have so far been filed many patents such as Japanese Patent Application Laid-Open No. Hei 3-252431, Japanese Patent Application Laid-Open No. Hei 3-252433, Japanese Patent Application Laid-Open No. Hei 4-224840, Japanese Patent Application Laid-Open No. Hei 5-51484 and Japanese Patent Application Laid-Open No. Hei 5-271477.
Further, with respect to a tire tread, it is known that the wet skid characteristic is improved by blending a rubber composition for a tire tread with silica. On the other hand, however, the defect that the abrasion resistance is lowered is involved therein at the same time. Accordingly, a rubber composition blended with a vulcanization-controlling silane coupling agent is reported as a composition for solving the defect of reducing the abrasion resistance in a silica-blended tread (Japanese Patent Application Laid-Open No. Sho 61-4742).
Further, known are a rubber composition comprising a polymer blended with a specific amount of carbon black and a specific amount of silica and silane coupling agent (Japanese Patent Application Laid-Open No. Hei 5-271,i ) and a rubber composition for a tire tread improved in an abrasion resistance and a wet skid characteristic by virtue of specific silica and specific carbon black and devising a in kneading process, and a production process for said rubber composition (European Patent No. 501227). Further, known is a technique of blending a rubber composition for a tread comprising a specific polymer and specific carbon black with silica in order to improve the wet skid characteristic and the abrasion resistance, (Japanese Patent Application Laid-Open No. Hei 7-48476, Japanese Patent Application Laid-Open No. Hei 7-90122 and Japanese Patent Application Laid-Open No. Hei 8-73657).
However, silica tends to cause coagulation between the grains thereof by virtue of hydrogen bonds among silanol groups which are surface functional groups, and therefore the kneading time has to be extended in order to improve the dispersibility of silica in rubber. The insufficient dispersibility of silica in rubber brings about a problem that the rubber composition is increases its Mooney viscosity and is inferior in processability such as extrusion.
Further, there has been a problem that since the surface of silica grains is acidic, the grains adsorb a base material used as a vulcanization accelerator in vulcanizing the rubber composition and therefore the vulcanization is not sufficiently carried out, so that the elastic modulus is not enhanced.
Further, there is another problem that since the dynamic storage elastic modulus (E') is lowered in a silica-blended composition, the block rigidity of the tire tread is reduced to deteriorate the dry performance. A pneumatic tire in which a rubber composition obtained by blending a specific polymer and specific carbon black with silica is applied to a tread is reported as improving the wet performance, the abrasion resistance and the dry performance (Japanese Patent Application Laid-Open No. Hei 8-53002). In such case, however, the abrasion resistance has not necessarily been sufficiently improved since a material having a high glass transition temperature (Tg) is used as a rubber component.
Various silane coupling agents have been developed in order to solve these problems, but the dispersion of silica has not yet reached a sufficiently high level, and it has been particularly difficult to obtain the industrially good dispersion of silica.
Further, it is disclosed in Japanese Patent Publication No. Sho 63-2886 and Japanese Patent Application Laid-Open No. Hei 6-157825 to use hydrophobic precipitated silicic acid. However, there has been involved a drawback that since precipitated silicic acid subjected to complete hydrophobicity treatment is used, surface silanol groups which react with silane coupling agents disappear, and therefore rubber is not sufficiently reinforced.
On the other hand, it is disclosed in Japanese Patent Application Laid-Open No. Hei 3-197536 to try to improve the heat generating property by blending 100 parts by weight of at least one rubber selected from the group consisting of natural rubber and diene base synthetic rubber with 0.05 to 20 parts by weight of an alkylamine compound as well as 20 to 150 parts by weight of a reinforcing filler such as carbon black. However, it is neither disclosed nor suggested in the application described above that an improvement in the dispersibility of silica has been tried for a rubber composition blended with silica and that a heat generating property and an abrasion resistance have been improved in the rubber composition blended with silica.
Accordingly, an object of the present invention is to solve the above problems involved in conventional techniques and to provide a rubber composition in which the dispersibility of silica is improved and a Mooney viscosity is lowered and in which the low heat generating property and the abrasion resistance are improved.
Further, another object of the present invention is to provide a rubber composition which can improve the wet performance, the abrasion resistance and the dry performance of a tire with a good balance thereamong when applied to the tread of the tire.